The present invention has as its object a feeler for an omnidirectional system comprising two supports, one of which is stationary while the other is movable and provided with a tip, a device for the relative positioning of the supports placed between them, comprising a plurality of positioning pieces connected to one of the two supports and a plurality of resting pieces connected to the other support in order to define a position of relative immobilization of these two supports by six point contacts spaced apart over 360.degree., and a spring to assure the six point contact engagement by action on the movable support, the six point contacts determining six planes of tangency between the positioning pieces and the resting pieces which are arranged oblique and secant in space, the movable support being capable of being displaced against the action of the spring to cause an interruption of contact between at least one positioning piece and one resting piece, and means being provided to detect the contact interruption.
This feeler is intended to be used more particularly but not, however, exclusively on machines for the three-dimensional measurement of an object such as, for instance, a machined mechanical part or a mechanical part in the course of machining with reference to a measurement system with three rectangular coordinate axes.
On these machines, the stationary support of the feelers is integral with a displacement device associated with the measurement system and intended to bring the tip into contact with an element of the object to be measured,. The position of this stationary support is detected by means of detectors with reference to the three coordinate axes and calculated by a computer.
When the tip enters into contact with the element of the object to be measured, any subsequent displacement of the stationary support of the feeler, which does not fail to take place due to the inertia of the displacement device with which it is integral, has the effect of causing the displacement of the movable support of the feeler to which the tip is fastened against the action of the spring and thus produce an interruption of contact between at least one positioning piece and one resting piece of the relative positioning device placed between the two supports.
This contact interruption, detected on these measurement machines by electrical means which produce an interruption signal, is used primarily to trigger the taking of a measurement, that is to say the taking of data for the computer which makes it possible to store the relative position of the stationary support reached at that precise moment. On certain measurement machines the interruption signal thus produced also serves automatically to interrupt the displacement of the stationary support of the feeler when the latter is remote controlled.
With respect to the triggering of the taking of the measurement, it is obvious that the precision of the measurement made is dependent to a great extent on the rapidity and precision of response of the device for the relative positioning of the two supports of the feeler. It is important, in particular, that this response be assured in all azimuths by the fact that on the machines the measurement is carried out in three dimensions by displacement of the stationary support in the directions of the three coordinate axes and even in certain recent designs by random displacements in the plane of two of these three axes.
In order assure this all-aximuth response, the designers use various systems known from metrology and, in particular, for some of them, the guidance systems of zero degree of freedom which make it possible to assure the relative immobilizing of the two parts by six contact points between them, such as the hole-line-plane fitting of Lord Kelvin or the Boys support which consists in resting three spherical feet of one part in three V-shaped grooves arranged in the shape of a star on the other part.
In this last system of immobilization in accordance with Boys, the walls of the three V-shaped grooves of the one part represent, at the six points of contact with the spherical feet of the other part, six oblique planes of tangency which are secant in space, distributed in pairs of two convergent planes 120.degree. from each other.
An example of the application of this immobilization system to a device for relative positioning between two supports of a feeler of a machine for three dimensional measurement is illustrated in U.S. Patent No. 4,270,275.
In a feeler structure described in that patent, six coplanar spherical resting pieces are arranged in three pairs, spaced 120.degree. apart, on a stationary support while three coplanar cylindrical positioning pieces are arranged in star shape 120.degree. from each other on a movable support with which there is associated a tip the end of which is spherical. A spring is placed under pressure on the movable support in order to maintain the three cylindrical positioning pieces resting against the three pairs of spherical resting pieces by six points of contact, at the level of which there are thus determined six oblique planes of tangency which are convergent in pairs between resting pieces and positioning pieces.
This system, which is self-adjustable, gives the feeler a triangular resting surface, the optimum being the distribution of the positioning pieces 120.degree. apart, forming a resting surface in the form of an equilateral triangle.
As a result, one approximates here the triangular characteristic of the resting surface of the feeler by a circle, minimizing the error of the method of least squares of a probability calculation.
In theory, it is obvious that the error introduced by this approximation could further be reduced by increasing the number of sides of the polygon of rest in order to arrive at a square resting surface. However, if one adds for this purpose a pair of resting pieces and a positioning piece to the known system, one obtains eight points of contact to assure the position of relative immobilization of the two supports, which makes the system hyperstatic and causes it to lose its self-adjustability feature.